In today's extensive centralized energy supply system, voltage control generally is effected by means of control transformers in the central nodes of the high or medium high voltage systems. For this purpose, the windings of the control transformers are provided with taps and it is possible to switch between adjacent winding taps without interruption under load by means of tap changers.
Generally, there exist two types of suitable tap changers: high-speed circuit breakers in which transition resistors are present and which can only be loaded during short periods of time for limiting the circuit current flowing during the switching operation and accordingly, effect a rapid switching between the winding taps, as well as those of the reactor switch type in which inductive transition impedances are used giving as a result a slow and continuous switching.
In the above-described voltage control in the field of high and medium-high voltage systems, it is however not possible to easily provide local control in distribution transformers in decentralized power-supply systems.
For this control that is effected close to the consumer in decentralized power supply systems, in particular in the USA, so-called “Voltage Regulators” have become widely accepted. Most common “Voltage Regulators” are single-phase, possess inductive transition impedances that are also referred to as reactor or reactor windings and enable 32-step voltage control, each step at ⅝%, i.e. in the range of +/−10%.
A different type of “Voltage regulator” is that of the Auto Boosters® type. This device has a less complicated structure and enables forward control in four steps of respectively 2½ or 1½%, i.e. +10 or +6% in total.
A further approach for providing voltage control that is close to the consumer in the field of low voltages is described in WO 2001/033308 [US equivalents U.S. Pat. Nos. 6,762,594 and 6,924,63] and WO 2003/044611 [US equivalent 20050017696]. Both applications in general are based on the object of providing a control transformer having a small number taps. Here, the individual partial windings are optionally looped by means of a changeover switch, the control transformer having a leakage impedance that is sufficient for limiting the circuit current to the order of the nominal current in the case of a short circuit of adjacent taps of the partial windings, which can occur during short periods of time when switching under load. The typical transition resistances of traditional tap changers can thus be avoided. In this arrangement, which is suitable for use as a control transformer of the autotransformer type or of a split-winding transformer type, different designs of the changeover switch are possible. Thus, it is proposed to use as changeover switch a load changeover switch of a tap changer that has no resistance contacts but only main contacts. According to other proposals, the changeover switch is designed as multiple cam stepping switch, optionally also composed of a series of relays or contactors, or finally, also consisting of a series of electronic switches, in particular thyristors. The number of possible positions thus corresponds to the number of required circuit elements of the changeover switch.
The disadvantage of this state of the art is that in particular in the case of the split-winding transformer, a separate primary and control winding must be provided. For raising the leakage inductance of each level such that the short-circuit current of the respective level only reaches the order of the nominal voltage, a short leakage channel is required. As a result a separate, short control winding is used and consequently leads to increased width and depth of the transformer. This additional expense of transformer costs is higher in many cases than gain obtained due to thus avoided transition resistances. Furthermore, the control performance is difficult; the known arrangement in particular is not suitable for parallel connections.